Concepts for mixing system

The objective was to generate three different concepts using the gas recirculation bubble piston method. Each concept’s main function was to generate a plug in a vertical draft tube based on the design criteria and the limitations set out. The three different concepts were evaluated in an evaluation matrix to find the best-suited option.

Concept 1

The first concept was based on a gas accumulator with a series of pressure vessels connected in parallel, where the compressed gas in the vessels is released in batches by automated valves to form a plug. A schematic process flow diagram of the concept can be seen in Figure 3.26 below.

Figure 3.26: Concept 1 – Pressure vessel with automated valve outside digester In this concept, the compressor supplies the accumulator with a constant flow rate and the accumulator ensures that the gas supply pressure stays constant.

49

The pressurised gas in the accumulator was then used to fill the two pressure vessels in batches by opening the inlet automated valves individually. As the pressure vessel reaches the set pressure, the inlet valve closes and the outlet valve opens. The pressurised gas is conveyed through a one-way valve before it reaches the set of nozzles at the bottom of the draft tube, creating a plug.

The set of nozzles located in the draft tube run the risk of becoming blocked. If this happens, the gas delivery line after the outlet automated valves has to be disconnected and flushed with highly pressurised water. It the pressurised water does not unblock the nozzles, the drawback of the concept is that the digester needs to be emptied and the nozzles have to be cleaned manually.

Concept 2

The second concept to generate a plug was derived from concept 1 by placing the gas accumulator at the bottom of the draft tube. This concept releases the gas plugs into the draft tube by a U-tube configuration. The process flow diagram for this concept can be seen in Figure 3.27 below.

Figure 3.27: Concept 2 – Gas accumulator with U-tube inside digester

The gas supplied from the compressor passes though the one-way valve into the gas accumulator at the bottom of the draft tube. This gas accumulator is open at the bottom and is connected with a U-tube to the draft tube. As the pressurised gas fills the gas accumulator, it displaces the digestate through the bottom of the gas accumulator and through the U-tube. When the gas has displaced all the digestate up to the lowest point of the U-tube, the gas escapes through the U-tube into the draft tube. As the gas escapes though the U-tube, the digestate level in the gas accumulator rises until it floods the U-tube and the cycle starts all over.

With this concept, the delivery pressure of the compressor will stay nearly constant, as the major factor determining the pressure is the hydrostatic pressure. The frequency of this mixing system can also be changed by just varying the gas supply flow.

50

The drawbacks of this concept are blockages of the gas accumulator and the U-tube. The gas accumulator can be flushed, but the U-tube, on the other hand, poses a problem as it cannot be flushed or pressurised inside the accumulator, as the bottom is open.

Concept 3

The third concept used the idea of the gas accumulator inside the digester, but consisted of a series of chambers that also worked on the U-tube principle. The aim with the series of channels was to decrease the chances of blockages compared to a single U-tube and to be able to flush the gas accumulator when needed.

The gas accumulator, also known as a plug-flow generator, was supplied with pressurised gas and released the accumulated gas into the draft tube in plugs. The flow diagram of the third concept is illustrated in Figure 3.28 below.

Figure 3.28: Concept 3 – Plug-flow generator

The pressurised gas supplied from the compressor first passes through a one-way valve and then to the plug-flow generator. The pressurised gas fills the plug-flow generator and displace the digestate out through the bottom of the open chambers.

When the displaced digestate level reaches the level just below that of the bottom of the first chamber (on the right-hand side in Figure 3.28), the accumulated gas escapes through this chamber and enters the draft tube as a plug. With the accumulated gas escaping in the first chamber, the digestate level in the third chamber (large chamber on the left-hand side) will rise to a point and the bottom of chamber two will be flooded.

In this concept there are no mechanical or moving parts in the digester. The system also keeps the load on the gas compressor constant and, by varying the gas flow rate, the frequency of plug formation changes. The series of chambers in this concept provides an advantage over the U-tube of concept 2, as they can be bigger in size and a high-pressurised water line can be incorporated with a set of nozzles to flush the plug-flow generator.

51

Concept evaluation

The three concepts generated for the mixing system were assessed in an evaluation matrix based on a selected set of installation, operation and maintenance criteria.

In the evaluation matrix criteria, an importance factor was assigned based on a scale from 1 to 5, with 5 being the highest. A rating was assigned to each criterion on a scale from 1 to 5, with 5 being the highest again. The importance factor and the rating giving to the concept were then multiplied to get a score. The scores of each concept were summed to get a total score for each concept, and these can be seen in Table 3.11 below.

Table 3.11: Mixing concept evaluation matrix

Evaluation criteria Importance Concept 1 Concept 2 Concept 3

Rating Score Rating Score Rating Score

Risk of blockage 5 5 25 2 10 4 20 Ease of cleaning 4 4 16 2 8 4 16 Maintenance 4 3 12 5 20 5 20 Manufacturing cost 3 1 3 5 15 4 12 Ease of installation 2 4 8 3 6 3 6 Design complexity 4 1 4 5 20 5 20

Operating live time 4 3 12 5 20 5 20

Total 80 99 114

Based on the concept elevation matrix presented above, concept 3 seemed to be the most promising option. This concept was used for the scale experimental. The numerical model to predict the mass displaced by the draft tube was also based on this concept.